As known, in a general knitting using knitting machines and the like, the yarn may be fed to the downstream machine by a so-called “positive” yarn feeder. With this type of feeder, the yarn is wound on a motorized, yarn-winding drum, which draws the yarn from a reel and feeds it to the downstream machine.
It is desirable to measure and control the yarn tension along the knitting line in order to maintain it substantially constant and to prevent surging of tension, which may cause defects in the finished clothes and affect the production yield. Since this tension depends on the difference between the speed of rotation of the drum of the feeder and the drawing speed of the downstream machine, it is conventionally controlled by modulating the speed of rotation of the drum on the basis of a signal received from a tension sensor arranged downstream of the feeder, by means of a tension control loop. In other words, the variation of tension to be applied is converted into a difference between the yarn-feeding speed and the yarn-drawing speed which is set on the downstream machine.
Although the above system effectively operates in steady state, a drawback well known to the person skilled in the art occurs in the transient state at the start of the feeding process, when the yarn is subjected to tension peaks due to the relatively low quickness of movement of the yarn-winding drum with respect to the high quickness of drawing of the downstream machine.
In order to overcome the above drawback, it is known, e.g., from EP 0 256 519, to provide the yarn feeder with a tension-limiting device capable of storing a reserve between the feeder and the downstream machine, which reserve is releasable during the starting transients in order to prevent the above tension peaks. In the embodiment of EP 0 256 519, a reserve is formed by deviating the yarn from its natural path between two stationary eyelets, by means of a rigid bar connected to the driving shaft of a motor. At rest, as well as in steady state, the bar is positioned in such a way as to deviate the yarn from its natural path. In the transient state at the start of the feeding process, the bar is rotated in such a way as to temporarily release the yarn.
The above tension-limiting device requires very accurate control of the movement of the bar and, therefore, introduces considerable complications in the control system of the feeder, with consequent rise in costs.
A simpler system, which is very effective in reducing the tension peaks, is described in U.S. Pat. No. 3,962,891, wherein, likewise the previous system, a reserve is formed by deviating the yarn from its natural path between two stationary eyelets. Unlike the previous system, however, in U.S. Pat. No. 3,962,891 the yarn is not deviated “actively” by a rigid bar connected to a controlled motor, but “passively” by an arm integral with a flat spiral spring which is loaded to a predetermined tension. During the starting transients, the arm bends in contrast to the returning action of the spring and the reserve is released. At this stage, the yarn is maintained at a desired level of tension depending on the preload on the spring.
The above system is effective and easy to put into practice, but it has the drawback that, when it is desired to adjust the operative tension in relation to any variations of the feeding tension—which, as known, are managed in a fully automated way on the basis of the characteristics of the yarn, of the type of processing, of the type of downstream machine, etc.—the load of the spring must be manually adjusted, with consequent reduction of the degree of automation of the line.